Optical determination of the material type of a sample is appealing in many applications, such as recycling, because it requires no contact. For opaque materials, such as metals, the bidirectional reflectance distribution function (BRDF) describes the optical response (reflection) of the material to incident light from different directions. This response may be used to characterize or classify materials. Because the BRDF is a 4-dimensional function of illumination (solid) angle and observation (solid) angle, typically only “slices” of it are measured, and a classifier is computed from these slices.
Some materials present themselves quite similarly to the classifier, and are thus difficult to classify by conventional means. As an example, metals of similar color can be difficult to distinguish from each other. FIG. 1 shows an example, in which the metals titanium, nickel and aluminum appear quite similar to one another, and are thus difficult to classify. Situations such as this may be challenging to conventional Computer Vision algorithms using RGB images only.